This invention relates to the energization of gas discharge lamps, and more specifically relates to a novel converter circuit for energizing and controlling the illumination output of gas-filled lamps and high intensity discharge lamps.
Illumination control systems are known and are shown in copending application Ser. No. 966,604, filed Dec. 5, 1978, entitled SYSTEM FOR ENERGIZING AND DIMMING GAS DISCHARGE LAMPS. This application is for an improved inverter for use with such systems.
To conserve energy in lighting applications using gas discharge lamps, it is known that the lamps should be energized from a relatively high frequency source, and that the lamps should be dimmed if their output light is greater than needed under a given situation. For fluorescent lamps, the use of a frequency of about 20 kHz will reduce energy consumption by more than about 20%, as compared to energization at 60 Hz. For high intensity discharge lamps, such as those using mercury vapor, metal halide and sodium, the saving in energy exists but is somewhat less than for a fluorescent lamp.
Energy saved by dimming gas discharge lamps depends on the degree of dimming which is permitted in a given situation. The light output of a lamp is roughly proportional to the power expended. Thus, at 50% light output, only 50% of the full rated power is expended.
Many applications exist where it is acceptable or desirable to decrease the amount of light from a lamp. For example, light in a building might be decreased uniformly or locally in the presence of sunlight coming through a window to maintain a constant or acceptable illumination at a work surface. Thus, during a normal work day, an energy saving of about 50% may be experienced. Light might also be decreased during non-working hours and maintained at a low level for security purposes. Light output might also be decreased, either from local controls or from a generating station during periods of overload on the utility lines.
Energy savings may also be obtained by dimming lamp output when the lamps are new and have a light output much higher at a given input power than at the end of their life. Since a lighted area must be properly illuminated at the end of lamp life, energy can be saved by dimming the lamps when they are new, and then reducing the dimming level as the lamps age. Energy savings of 15% for fluorescent lamps and 20% to 30% for high intensity discharge lamps can be obtained in this fashion.
One system used at the present time to obtain the benefits of high frequency energization of gas discharge lamps distributes power at low frequency (60 Hz) to each of the fixtures of a lighting system. Each fixture could commonly contain several lamps in parallel or series connection. Each fixture is also provided with an inverter to produce the high frequency energizing power and contains the necessary ballast circuits for the lamp. Circuits used in the individual fixture for the above type circuit are typically shown in U.S. Pat. Nos. 3,422,309, 3,619,716; 3,731,142; and 3,824,428, each in the names of Spira and Licata; and 3,919,592 in the name of Gray, each of which is assigned to the assignee of the present invention. Systems of this type are available from the Lutron Electronics Co., Inc. of Coopersburg, Pa. under the trademark Hi-Lume.
While the above arrangement performs well, a complete inverter circuit and controls therefor must be placed in each fixture. Thus, the system is costly and the reliability problem is repeated for each fixture. Since each fixture receives the complete inverter circuit, designers and users are hesitant to use complex and expensive circuits and control schemes because of cost and reliability. Furthermore, each circuit exists in the hot, hostile environment of the lamp fixture. The scheme also requires that four leads go to each fixture; two for power and two for the dimming signal. A further problem is that it is difficult to provide a good 50 Hz to 60 Hz power factor in each fixture since the power factor correction devices are bulky and expensive.
In another known system, a single source of high frequency is used and provides energy for a relatively short distance over relatively short power lines. Dimming is obtained by changing the inverter frequency to a capacitive ballast.
This arrangement has several disadvantages. First it provides relatively poor dimming. The lamps used in the system require separate filament transformers since, if high frequency is used to power the filaments, it is difficult to keep the filament voltage constant with variable frequency. The separate filament transformers are costly and further complicate the system. It is also difficult to change the inverter frequency and requires costly and complex controls. A further problem of these systems is that the load on the inverter is capacitive so that the high frequency power factor is poor. Thus, excessive current flows in the wires between the inverter and ballast, creating additional energy loss.
Other arrangements are known in which 50 Hz to 60 Hz power is supplied from a local source directly to the lamps and their ballasts, and dimming is obtained by changing the current amplitude through the use of an auto-transformer or thyristor control circuit. While this system obviously does not have the advantage of high frequency excitation for the lamps, it is also true that bulky components are needed in this fixture and a good 50/60 Hz power factor is hard to obtain.